This invention generally relates to optical amplifiers and is specifically concerned with a simpler and less expensive control system and method that requires fewer signal monitoring components.
Erbium doped fiber amplifiers (EDFAs) are used in optical transmission networks to extend transmission distances and to compensate for losses from various network elements. Such amplifiers typically comprise a pair of pump lasers whose outputs are optically coupled to the inputs of two, serially connected coils of erbium-doped optical fiber. In operation, the outputs of the pump lasers excites the atoms of erbium dopant within the serially connected coils of fiber. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output.
When such EDFAs are used simply as amplification relay stations along a single, long-distance optical circuit, there is little need for a capacity to specifically control the amount of gain that the amplifier imparts on the incoming optical signal, as there is typically little change in the strength of the incoming signal. However, as optical systems have become more complex, the need for such a gain controller in the amplifier has increased. Such a need may arise, for example, when an optical network is installed around an urban area. Under such circumstances, the distances between the optical amplifiers may be very different, If the EDFAs in the system all have the same amplification capacity, this capacity must be adjusted by way of a gain control device so that the signal strength remains uniform throughout all branches of the network.
More recently, there has been a growing demand for optical gain controllers which are capable of maintaining a preselected gain setpoint despite rapid variations in the strength of the incoming signal. Such a control system is needed in optical networks transmitting dense wavelength division multiplexed signals (DWDM), wherein a plurality of different optical channels are being periodically added and dropped. Such a control system needs to maintain the selected gain setpoint over a broad dynamic range despite signal strength transients generated by the adding and dropping of channels. It further needs to uniformly amplify each channel, or to cause each channel to be uniformly amplified in the system by having a selectively tilted gain spectrum that compensates for under-amplified channels in the input. This requirement is referred to as gain flatness. Otherwise, such under-amplified channels may become lost at a point downstream in the network. Finally, the control system must have good transient characteristics. When additional channels are added or dropped, the total optical power may experience large upward or downward transient spikes that may last up to a millisecond. These spikes may cause a temporary increase in the bit-error-rate.
To meet this demand, an EDFA having a flat gain response with good transient characteristics was developed by Corning, Incorporated of Corning, N.Y. Such an optical amplifier is illustrated in FIG. 1, and disclosed and claimed in parent U.S. patent application Ser. No. 09/680,021, filed May 17, 2001. Such an amplifier generally comprises a pair of amplifier stages serially connected by a variable optical attenuator which operates to create the desired flatness or desired tilt in the output. As will be described in more detail hereinafter, the control system of such an amplifier operates by monitoring the strength of the optical signal both before and after each of the two coils of erbium-doped gain fiber. Each of the four monitoring circuits comprises an optical tap which diverts some of the light conducted through the amplifier to a photodiode, which in turn converts this light into an electrical signal. A transimpedance amplifier is connected to the output of the photodiode. The output of each of the four transimpedance amplifiers is conducted to the input of a digital signal processor, which proceeds to maintain a preselected gain setpoint by adjusting the amount of power conducted to each of the two pump light sources in response to the signals received from each of the four transimpedance amplifiers.
While the control system for the aforementioned amplifier is capable of dynamically maintaining a gain setpoint over a broad range and with a relatively flat output and good transient characteristics for the different channels being amplified, the inventors have noted some aspects of the design of this control system which might be improved. In particular, it would further be desirable if at least one of the monitoring circuits in the amplifier could be eliminated, as each such monitoring circuit requires relatively expensive, precision circuitry, and further weakens the gain capacity of the amplifier due to the necessary diversion of optical signal. It would be desirable if a dynamic controller for an optical amplifier could be developed which maintained all of the desirable response characteristics of the controller illustrated in FIG. 1, but which was simpler and less complicated in structure.
The invention is a variable gain optical amplifier that overcomes the aforementioned disadvantages of previously designed amplifiers. To this end, the optical amplifier of the invention comprises at least one amplifier stage having a light pump, and a power source for the pump; a variable optical attenuator connected to the amplifier stage and having a movable controller that changes attenuation of an amplifier output when moved to a different position, and a dynamic controller that maintains a selected gain setpoint for the amplifier. The dynamic controller includes gain detecting circuits that generate signals indicative of input signal and output signal strength of the amplifier stage, and a circuit that provides a signal indicative of a position of the attenuator controller. The dynamic controller further includes a signal processor connected to the gain detecting circuits and the position indicating circuit. The signal processor maintains a selected gain setpoint for the amplifier in accordance with signal input and output strengths of the amplifier stage, and a predetermined relationship between a position of the attenuator controller and signal attenuation.
The signal processor may be connected to the pump power source of the amplifier stage, and may maintain the selected gain setpoint by modulating power from the power source in response to signals from the gain detecting circuits and the position indicating circuit. The signal processor may also be connected to the attenuator controller and may maintain the selected gain setpoint by adjusting the movable attenuator controllers.
In a preferred embodiment of the invention, the signal processor maintains the selected gain setpoint by means of a look-up table correlating a selected gain setpoint with input and output signal strengths of the amplifier stage, and a position of the attenuator controller. Alternatively, the signal processor may operate by means of a preprogrammed formula or algorithm that correlates these parameters.
The variable gain optical amplifier may also include a second amplifier stage having an input that is connected to an output of the first stage via the variable optical attenuator. This second amplifier stage may also include a light pump and a power source therefor which is modulated by the digital signal processor. In such an embodiment, an additional gain detecting circuit is provided at the output of the second stage, and the signal processor maintains a preselected gain setpoint by means of a predetermined relationship between the outputs of the three gain detecting circuits and the position indicating circuit of the attenuator controller.
The invention also encompasses a method of controlling a variable gain optical amplifier of the type having an amplifier stage connected to a power source, and a variable optical attenuator having an input connected to an output of the amplifier stage and a movable controller that changes signal gain when moved. The method comprises the steps of monitoring the strength of an incoming signal transmitted to an input of the amplifier stage, monitoring the strength of an attenuated, amplified signal transmitted from an output of the variable optical attenuator in accordance with a predetermined relationship between a position of the movable controller and signal attenuation, and maintaining a predetermined gain setpoint by varying the amount of power conducted to the amplifier stage from the power source and/or varying the position of the movable controller of the optical attenuator in response to changes in the strength of the input and output signals.
The invention provides an optical amplifier having a dynamic controller capable of providing flat output gain over a broad range of gain with good transient characteristics by means of a simpler controller that replaces a gain detecting circuit with a relatively inexpensive circuit that provides a signal indicative of a position of a controller for a variable optical attenuator.